The present invention is directed to an OFDM/DMT digital communications system. More particularly, the present invention is directed to an apparatus and method for attaining symbol alignment in a multi-point, OFDM/DMT digital communications system.
Multi-point communications systems having a primary site that is coupled for communication with a plurality of secondary sites are known. One such communications system type is a cable telephony system. Cable telephony systems transmit and receive telephone call communications over the same cable transmission media as used to receive cable television signals and other cable services.
One cable telephony system currently deployed and in commercial use is the Cablespan 2300 system available from Tellabs, Inc. The Cablespan 2300 system uses a head end unit that includes a primary transmitter and primary receiver disposed at a primary site. The head end unit transmits and receives telephony data to and from a plurality of remote service units that are located at respective secondary sites. This communication scheme uses TDM QPSK modulation for the data communications and can accommodate approximately thirty phone calls within the 1.9 MHz bandwidth typically allocated for such communications.
As the number of cable telephony subscribers increases over time, the increased use will strain the limited bandwidth allocated to the cable telephony system. Generally stated, there are two potential solutions to this bandwidth allocation problem that may be used separately or in conjunction with one another. First, the bandwidth allocated to cable telephony communications may be increased. Second, the available bandwidth may be used more efficiently. It is often impractical to increase the bandwidth allocated to the cable telephony system given the competition between services for the total bandwidth available to the cable service provider. Therefore, it is preferable to use the allocated bandwidth in a more efficient manner. One way in which the assigned bandwidth may be used more efficiently is to use a modulation scheme that is capable of transmitting more information within a given bandwidth than the TDM QPSK modulation scheme presently employed.
The present inventors have recognized that OFDM/DMT modulation schemes may provide such an increase in transmitted information for a given bandwidth. Such systems, however, present a number of technical problems. One such problem is the determination of how a newly added receiver is to attain symbol alignment without disrupting communications to the other receivers that are already in communication with a primary transmitter at a central site. A remote receiver must first obtain symbol alignment to properly recognize the data that is transmitted from the primary transmitter before it can perform the functions required of it. Also, a newly added transmitter must align its symbol transitions with other remote transmitters. The present inventors have recognized that need and have disclosed a solution to the problem.